The present invention relates to devices for measurement of electrical circuits and particularly to probes for making direct, temporary contact to selected points on microcircuits.
Microcircuit probes are used to provide temporary electrical connections between test equipment and selected points, usually bonding pads, on integrated circuit (IC) devices. A microcircuit probe should provide good electrical contact without applying excessive pressure to the IC device. Also, the probe should present a constant characteristic impedance path from the bonding pad of the microcircuit to the point of connection of the probe with the measurement equipment, particularly when used in measurement of IC devices operating in the microwave frequency range. A discontinuity can cause unwanted signal reflections at the point of discontinuity resulting in inaccurate measurements.
In the prior art, the problem of making good electrical contact between the probe and the microcircuit bonding pad has been solved by attaching a small length of spring wire at the probe tip to make the actual contact with the bonding pad. This approach is often used when a probe is intended to make more than one contact to the microcircuit particularly when the bonding pads to be probed are at differing elevations. The spring wires establish good contact to all bonding pads while their resilience prevents the probe from applying undue pressure to any one bonding pad. Unfortunately the characteristic impedance of a spring wire is usually much different than the impedance of the rest of the probe signal path and at higher frequencies the spring wire can cause unwanted reflections.
Therefore what is needed is a probe providing a path of constant characteristic impedance throughout the length of the probe from the point of connection of external test equipment to a point as close as possible to the microcircuit test point. The probe should also maintain good electrical contact while exerting limited contact pressure.